Medicago truncatula interaction with insects

Medicago truncatula is attacked by a broad range of insect herbivores including sieve-element feeders, cell content feeders, and chewing insects. In M. truncatula, genetic resistance against insects has to date been identified only against aphids: single dominant resistance genes have been identified conferring resistance against the pea aphid, bluegreen aphid, and spotted alfalfa aphid. Aphids are readily maintained in the laboratory because of their clonal reproduction, and because they can be maintained on intact plants, excised leaves, or on artificial diet. Aphid host plant resistance studies should include host selection assays, measures of aphid growth and development, and also effects on plant vigor. Direct measurements of aphid feeding behavior can also be accomplished using the electrical penetration graph (EPG) technique. Recent studies of aphid-induced gene expression in M. truncatula implicate the octadecanoid pathway in resistance against the bluegreen aphid. Bioassays of cell content feeders and chewing insects generally focus on the performance of the herbivores, measuring individual development or population growth. Defence gene expression studies have also implicated jasmonic acid signaling and the octadecanoid pathway in defence against chewing insects. Calcium oxalate crystals may also function in chewing insect defence. 1 Overview of insects attacking Medicago truncatula Medicago truncatula, like other legumes, is host to a broad range of arthropod herbivores (Edwards and Singh 2006). For example in Australia, where it is grown as a pasture species, a number of insect and mite species are recognised as pests on M. truncatula. In the establishment phase, seedlings are attacked by redlegged earth mite (Halotydeus destructor), the blue oat mite (Penthaleus major), bluegreen or blue alfalfa aphid (Acyrthosiphon kondoi), spotted alfalfa aphid (Therioaphis trifolii maculata), sitona weevil larvae (Sitona discoideus), and the brown pasture looper (Ciampa arietaria). The bluegreen aphid, sitona weevil adults, and the lucerne flea (Sminthurus viridis) can also cause serious damage when the plants are more established. Other insect species with more generalist feeding habits can be occasional pests, such as the pasture day moth (Apina callisto) and the cotton bollworm (Helicoverpa armigera). Other species, such as Spodoptera littoralis, known to feed on relatives of M. truncatula also feed successfully on M. truncatula under laboratory and glasshouse conditions. The assemblage of herbivores attacking M. truncatula include sieve-element feeders (aphids), cell content feeders (mites, lucerne flea), and chewing insects (sitona weevil, moth larvae). Thus, M. truncatula should be a useful model to study mechanisms of defence in legumes against all three feeding strategies. However, to date genetic resistance has been identified only against aphids in M. truncatula. This has produced cultivars with resistance to two cosmopolitan aphid genera that are especially important pests of Medicago spp.: Acyrthosiphon (including bluegreen aphid and pea aphid) and Therioaphis (including spotted alfalfa aphid and spotted clover aphid). The presence of genes for aphid resistance in multiple genetic backgrounds provides a useful system for discovering mechanisms of defence against these insects (Klingler et al., 2005; Gao et al., 2006a; Gao et al., 2006b). Defence against chewing insects has also been studied in the M. truncatula reference genotype A17 (Leitner et al., 2005; Bede et al., 2006) and in mutants deficient in calcium oxalate crystals (Korth et al., 2006). Medicago truncatula interaction with insects page 2 of 15 Medicago truncatula handbook version November 2006